Peel and section orange over a bowl; squeeze membranes to extract juice. Reserve juice for another use; discard membranes.
Combine orange sections, sweetener, and grapefruit in a blender or food processor; process until smooth. Pour mixture into a 13 x 9-inch baking pan. Cover and freeze 2 hours, stirring every 30 minutes with a fork. Let stand 5 minutes before serving. Garnish with mint sprigs, if desired.